In an image acquisition device for observing a sample such as a tissue cell, when the distance between the sample on the stage and an objective lens is kept constant, irregularities on a surface of the sample may have an out-of-focus region mixed in an image. Therefore, image acquisition devices employing various focusing methods such as a dynamic focus scheme which captures an image of the sample while acquiring focus information and a prefocus scheme which acquires focus information before capturing the image of the sample have been developed.
Known as an example of such image acquisition devices is a microscope device disclosed in Patent Literature 1. This device is equipped with an image pickup element, drivable in a rolling shutter mode, having a plurality of pixel lines. A light source is caused to emit light intermittently at an interval longer than the light-receiving time of each pixel line, and focal position information of a microscope optical system is acquired according to images captured while moving a stage.
A slide scanner disclosed in Patent Literature 2, for example, continuously reciprocates the focal position of an objective lens while moving a field position of the objective lens with respect to a sample, and acquires image data of the sample by using a line scan camera. This slide scanner calculates an in-focus position of the sample according to the height of a contrast value of the acquired image data for each predetermined region, thereby producing a focus map of the sample as a whole.